sched: add mutex lock to protect qos_level

jingdong inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I80FQN



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In order to reduce the concurrent operations introduced
by users setting qos_level, we add a mutex lock.

Fixes: bda15404 ("sched: Introduce qos scheduler for co-location")
Signed-off-by: zhangwei123171 <zhangwei123171@jd.com>
Reviewed-by: zhaoxiaoqiang11 <zhaoxiaoqiang11@jd.com>